This invention relates generally to extended foil capacitors and, more specifically, to an improved end electrode for extended foil capacitors.
A capacitor is an electronic component for storing electrical energy. In a typical capacitor configuration, two large conductive surfaces are spaced apart by a thin dielectric material. While these surfaces may be planar, they are often rolled and placed in a cylindrical can to save space and provide physical protection. In order to obtain an optimal electrical connection to each rolled surface and thereby minimize internal impedance, the "extended foil" capacitor was developed. This capacitor has a capacitance section formed by convolutely rolling about a mandrel several stacked foils. This stack typically includes three electrical layers sandwiched around first and second conducting layers. One edge of one conducting layer extends from one longitudinal end of the section; the opposite edge of the other conducting layer extends from the other end. These ends are the points of electrical connection to the spaced conductive surfaces of the capacitor.
The reliable connection of external electrodes to the extended conductive ends has been an ongoing problem with extended foil capacitors.
U.S. Pat. No. 2,539,332 of W. G. Schneider discloses an extended foil capacitor having a screen electrodes mechanically held against extended ends 23 of the foil.
U.S. Pat. No. 2,805,372 of W. G. Schneider discloses an extended foil capacitor having a metal plate 25 with sharp edges on radial slots 26 to cut through wax that impregnates the capacitor section and ensure mechanical contact with extended foil 22.
U.S. Pat. No. 4,685,027 of G. Mouries discloses an extended foil capacitor with a "schooped" brass plate held in contact with the foil by pressure.
Reliability may be a problem with capacitors using physical pressure to maintain contact. If the foil moves relative to the electrode, the foil may break or internal shorts may occur. It is difficult to solder or weld electrodes to the aluminum foils often used in capacitors. Because of this difficulty, applicant had been using a conductive paste such as silver filled epoxy to bond the extended foil ends to a screen similar to that shown in the aforementioned Schneider '332 patent. The holes in the screen permit the application of the paste after the capacitor is assembled. The paste is blocked by the tightly wound dielectric layers from shorting the other conducting surface.
This construction is found to have at least two major problems. The perpendicular screen conductors do not provide a symmetrical current path from the circular foil edges, an important consideration in high current applications. And the radially outer cut ends of the screen wire form sharp projections which tend to cut through surrounding insulation under vibration.